Conventionally, some exposure units (exposure heads) of image forming apparatuses (printers, electrophotographic devices, etc) employing the electrophotographic process comprise organic EL elements as light emitting elements serving as the light source.
Recently, intensive efforts have been made to develop plane emission devices using organic EL elements. By using organic EL elements in place of LEDs as the light source in electrophotographic process, the number of wire bonding connections to LEDs can be reduced, which will significantly contribute to reduced cost.
However, under the present circumstances, organic EL elements are subject to notable reduction in light quantity due to deterioration with time compared to LEDs and, accordingly, uneven luminance tends to occur. For this reason, when organic EL elements are used in the exposure unit of an image forming apparatus, uneven print density tends to occur due to deterioration with time.
Then, a light reception element is provided along with each organic EL element. The light reception element monitors the light from the organic EL element. Drive signals are controlled so that each organic EL element has an emission luminance of a predetermined value, thereby correcting any reduction in light quantity due to deterioration with time and preventing uneven luminance, which consequently prevents uneven print density of an image forming apparatus in which the exposure unit comprises organic EL elements. Such a structure is disclosed, for example, in Unexamined Japanese Patent Application KOKAI Publication No. 2002-144634.
Organic EL elements tend to be subject to deterioration with time, namely the emission luminance drops with time when a constant voltage is applied. Therefore, in the structure as disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2002-144634, the voltage value of the drive voltage applied to the organic EL elements is progressively increased to maintain the emission luminance of the organic EL elements at a fixed value regardless of deterioration with time.
However, the deterioration with time of organic EL elements is accelerated as the voltage value of the applied voltage is increased. Therefore, when only the voltage value of the drive voltage is controlled as in the above case, the deterioration with time of organic EL elements is accelerated. Consequently, the organic EL elements will have a short life.